The present invention relates to a novel process for making a rhodium-plated article developing black color and having an improved wear-resistance.
All the colors of the platings which are practically obtainable through known rhodium-plating baths are white, the known rhodium baths comprising various rhodium salts, e.g., sulfate or phosphate.
Rhodium-plating has been generally employed for use in electric contacts, lead frames and ornamental articles due to its good hardness, wear resistance, corrosion or erosion resistance and stability in the contact resistance. Furthermore, it has been desired to develop rhodium plating with different colors besides white, as it will enhance the practical value of the rhodium plating.
On the other hand, known manners for black plating or black coloring such as black chrome plating, black nickel plating, blackening with copper carbonate all encounter difficulties in obtaining gloss, some of them yielding clouding, that is to say, these all being less valuable in the ornamental effect. There have been further problems in the manufacturing process, wherein they showed bad reproducibility in operation conditions, difficulties in mass production or the like. Thus there has been much to be desired in the prior art and it has long been desired to develop black color plating of noble metal in the ornamental article industries.